Method and apparatus for reclaiming metal values from electric arc furnace flue dust and sludge and rendering residual solids recyclable or non-hazardous

ABSTRACT

A method for the pyrometallurgical treatment of environmentally hazardous steelmill flue dust and sludges to separate and recover the volatile heavy metals from the flue dust material and to convert the remaining iron into recyclable directly reduced iron pellets, or disposable non-hazardous reoxidized iron pellets, or pig iron or shot and slag. The dust is mixed with solid carbonaceous reductant and a binder, pelletized, optionally dried, and reduced in an inclined rotary reduction smelter vessel. The distilled heavy metals are reoxidized and may be selectively segregated to reduce the gangue and lead oxide contamination from the reconcentrated zind oxide dust. The reconcentrated heavy metal oxide dust is recycled by the zinc/lead industry.

BACKGROUND OF THE INVENTION

Direct reduction of iron-oxide ore in fine, lump, and pellet form iswell known in metallurgical literature and patents for processesinvolving rotary kilns, shaft furnaces, fluidized beds, and retorts ofvarious sizes and shapes. In general, such processes have evolved forthe purpose of directly reducing ironoxide ore in solid-state form tometallized iron, commonly known as directly reduced iron, or "DRI",which can be easily melted in an electric arc furnace to produce highquality steel, having a low percentage of residual elements, or gangue.

"Metallized", as used throughout this specification does not mean coatedwith metal, but means nearly completely reduced to the metallic state,i.e., always in excess of 60% metal, and usually in excess of 90% metalin the material. Such metallized iron in many forms, including pellets,is well suited as feed material to steelmaking furnaces such as anelectric arc furnace.

Existing direct reduction processes are generally intended for hightonnage production rates on a continuous basis, with the end productbeing high grade DRI with metallization levels about 92 to 94 percent,and having a maximum residual gangue content of less than eight percent.While rotary kilns, in general, utilize solid forms of carbon such aslignite, coal, or coke to provide the reductant source, shafts furnaces,retorts, and fluidized bed furnaces normally utilize natural gas or oilto provide the reductant source.

Recent advancements in plasma melting technologies have resulted in thedevelopment of several new smelting processes capable of directlyreducing and melting iron-oxide ore to produce pig iron or steel. Thesenew processes for the most part are still in the experimental stage ofdevelopment, and are dependent on low power costs to be economical.Plasma processes in the field of electric arc furnace flue dustreprocessing are primarily designed to recover only zinc and lead incrude metallic form while the iron contained in the material is meltedin oxide form and becomes inseparable from the slag which must then bedisposed of as solid waste.

Existing direct reduction processes designed to produce large annualtonnages of DRI for use as a remelt stock in electric arc furnacessteelmills, in general, cannot be economically downsized to meet theneeds for "on-site" reprocessing of small tonnages of flue dustmaterial. Recent development of direct reduction and direct smeltingprocesses such as rotary kiln, rotary hearth, retort, and plasmafurnaces each have possible on-site application where the economy-ofsale is favorable. An apparatus and process which provides distillationand recovery of the heavy metal oxides contained in electric arc furnaceflue dust as well as recovering metallic iron in the form of either DRI,or pig iron from the resulting slag on a small scale and at a lowcapital and operating cost would be beneficial. The majority ofmini-steelmills, both in the United States and in the rest of the world,produce less than 10,000 tons per year of flue dust each.

The Resource Conservation and Recovery Act of 1976 ("RCRA"), and the NEWRCRA Hazardous and Solid Waste Amendments of 1984 for handling anddisposing of waste materials listed by the U.S. Environmental ProtectionAgency as hazardous, establishes certain guidelines and deadlines withwhich producers of hazardous waste materials must be prepared to complyby Oct. 8, 1988. The Comprehensive Environmental Response, Compensation,and Liability Act ("CERCLA") establishes potential liabilities forclean-up of existing hazardous waste disposal sites and inorder toensure compliance with these regulations as well as future regulations,and to avoid potential future liabilities, it is imperative that allproducers of hazardous waste materials in "on-site" facilities in thefuture.

In view of existing technology available, and the economics of applyingsuch technology, there remains the need for simple, low cost, safe, andeffective processes for the treatment and conversion of waste materialsclassified as hazardous by the EPA into recycable or non-hazardousdisposable materials.

BRIEF DESCRIPTION OF THE INVENTION

This invention is a pyrometallurgical apparatus and process for thedirect reduction of iron oxides, and the reduction, selectivedistillation, reoxidation, and recovery of volatile heavy metals (inoxide form) commonly found in steelmill electric arc furnace (EAF) fluedust. While the invention has the capability of effectively reducing anyiron oxide form, such as naturally occuring iron ore, mill scale, blastfurnace and BOF dust, the invention is specifically designed to meet theeconomic needs for "on-site" application at ministeel mills.

The invented process comprises the steps of preparing green ball pelletsfrom flue dust and/or sludge, drying the pellets if necessary, heatingand reducing the pellets in a rotary reduction smelter, melting thereduced pellets within the smelter, refining the molten, metallizedmaterial, and pouring or casting the molten material. An additionalfeature of the invention is the recovery of metal values from theoff-gases from the smelter.

OBJECTS OF THE INVENTION

It is the principal object of this invention to provide a small-scaleflue dust recovery plant that can be operated on a discontinuous basis,and without costly degradation to the systems refractories.

It is another object to provide economic application "on-site" atmini-steelmills for the purpose of recycling hazardous EAF flue dustinto recyclable, and/or non-hazardous materials, such as directlyreduced iron (DRI) pellets, or pig iron and slag, and a highlyreconcentrated zinc and lead oxide dust/ore.

It is also an object to employ greenball pellets containing admixedsolid carbon reductant directly in a hot rotating furnace withoutcatastrophic degradation of the pellets, and dust regeneration.

Another object is to provide a method of operating the invention as abatch process to selectively distill and recover reconcentrated zincoxide dust, lead oxide dust, and process gangue dust separately.

It is also an object to provide apparatus having sufficient operatingflexibility to allow feed material to be converted to solid-statedirectly reduced iron (DRI), or to be melted to form liquid iron andslag, while distilling volatile heavy metals for subsequent recovery inan off-gas cooling and dust collection system.

It is also an object to provide means to convert hazardous waste fluedust which has no direct commercial value into marketable products andby-products, and non-hazardous waste materials that can be safelydisposed of according to the EPA EP Toxicity Regulations.

It is another object of this invention to provide a variable operatingfurnace apparatus which can be selectively controlled either to melt themass of feed material to form liquid iron and slag, or to stop short ofmelting and produce a solid state iron pellet, which can be recycled asremelting stock into an electric arc furnace.

It is also an object to provide means to terminate the existence ofcertain flue dust and sludge materials as hazardous waste.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is better understood by referring to the followingdetailed description and the appended drawings, wherein:

FIG. 1 is a process flow diagram illustrating the relationships of thevarious pieces of apparatus employed in this invention.

FIG. 2 is a side elevation of the inclined rotary reduction smelter ofthe invention.

FIG. 3 is a feed-end view of the inclined rotary reduction smelter ofFIG. 2, with off-gas fume hood in place, and including the off-gasafterburning and gas cooling apparatus.

FIG. 4 is a side elevational view in cross-section of the inclinedrotary reduction smelter apparatus, further illustrating the batchcharging apparatus during batch charging operation.

FIG. 5 is a side elevational view in cross-section similar to FIG. 4,further illustrating the positions of the invented apparatus with thefume hood in the operating position, with the batch charging apparatusretracted, and with the retractable burner in the smelter heat-upposition.

FIG. 6 is a side elevation view similar to FIG. 4, showing the rotaryreduction smelter in the operating position with the retractable burnerin the pilot position, and further showing the rotating burner and gasinjection system operational.

FIG. 7 is a side elevational view in cross-section view of the inventionin operating position with gas injection underbed to melt down batchcharge.

FIG. 8 is a side elevational cross-section of the rotary reductionsmelter in position to discharge either solid directly reduced ironpellets, or liquid iron and slag, the latter being illustrated.

FIG. 9 is a cross section of a portion of the rotary reduction smelter,on larger scale, illustrating the gas injection apparatus, with a fixedposition centerline burner and tuyeres located in the sidewall of therotating hearth.

FIG. 10 is a diagramatic illustration of the process including a rotaryhearth direct reduction apparatus in the flowsheet.

DETAILED DESCRIPTION

Referring now to the drawings, and particularly to FIG. 1, the processflow and apparatus relationships are illustrated. Electric arc furnace(EAF) flue dust from the EAF dust collecting system or storage bin 10 isdelivered by screw or pnuematic conveyors 12 to a flue dust day bin 14.Coal or coke from solid reductant bin 16 provides the principalreductant for the process. A binder for making a strong greenballpellet, preferably an organic, cellulose-based colloidal binder, isstored in binder bin 18.

In order to pelletize greenballs, raw materials from bins 14, 16, and 18are metered onto screw conveyor 20, which transports the materials tomixer 22. The reductant feed rate to the mixture is determined by theiron (Fe) content of the EAF dust. It has been determined by practicethat for each pound of iron, 0.35 to 0.50 pounds of fixed carbon isrequired, depending on the desired end product. Feed weights from eachbin are controlled by load cells connected to each bin. During mixing,approximately ten to thirteen percent water is added to achieve properpelletizing composition, depending on the nature of the flue dust andthe type of reductant used in a particular plant. The mixed material isconveyed from the mixer 22 to pelletizing disc 24 by which greenballsare formed. Both the mixer 22 and pelletizing disc 24 are commerciallyavailable apparatus.

When properly controlled, the pelletizing disc 24 acts as an autosizer,and no screening is required prior to the pellets being fed to pelletdryer 26. Air preheated to approximately 300° C. by a heat exchanger 28in waste gas afterburner 30 is blown and sucked through the pelletdryer, first in a downdraft direction and second in an updraftdirection. The pellet dryer 26 and subsequent batch charging system 32is sized to hold the live load of a unit charge. This prevents flowproblems inherent with greenball pellet holding bins. The pellet dryerapparatus 26 is unique, and intended for application only in associationwith the process of this invention. In certain situations, the pelletdryer may not be required in the process and may be omitted.

The batch charging system 32 contains a unit charge of previously driedpellets. When the demand is made for a new batch, the system will conveya properly sized batch of pellets onto the preheating hearth 36 of theinclined rotary reduction smelter 38. Retractable feeder 40 moves intothe charge or feed position in the preheat hearth 36 of the smelter anddischarges the unit batch into the smelter.

The inclined rotary reduction smelter 38, as illustrated in FIGS. 2 and4 through 8, is a multi-function vessel, having the abilities to rotateat various speeds and tilt to positions A and B to discharge its burdenand take on a new batch respectively. When a new batch is introducedinto the smelter, the rotation rate is reduced to a slow speed to allowminimum rolling of the pellet bed while the material is in thepreheating position. As soon as the unit batch is placed on the preheathearth 36, the smelter returns to operating position C, the movable fumehood 42 is returned to operating position covering the smelter chargeopening 44, the retractable burner 46 is inserted to the position shownin FIG. 6 and fired in oxidizing mode to rapidly raise the temperatureof the batch to about 600° C. to complete devolatilization of theadmixed coal (reductant) in the pellets, and reduction of the metaloxides begins. The smelter rotation speed is slowly increased to rotatethe bed and move the batch down to the operating position. The rotationspeed is controlled to prevent the pellet batch from sticking to the hothearth walls.

The gas feed to retractable burner 46 is controlled to maintain aslightly oxidizing atmosphere as smelter off-gases are monitored forexcess oxygen content while the heat-up of the batch continues. When thebatch temperature reaches about 900° C., and as reduction of metaloxides increases, the retractable burner gas feed is adjusted to aslightly reducing atmosphere while the batch temperature is raised to1050° C. to 1150° C.

When the rate of oxide reduction decreases, the exit gas from thesmelter again becomes oxidizing and the reduction process is complete.The process can be stopped at this point by decreasing the gas feed tothe retractable burner 46, and retracting it into the movable fume hood42. The fume hood is then removed to clear the mouth of the smeltervessel, and the smelter is tilted to casting position from which theburden is dumped into a rotary cooler 50, in which the coolingatmosphere is controlled to produce either metallized pellets orreoxidized pellets. Before the movable fume hood 42 is retracted fromthe operating position, the smelter and afterburner atmospheres must beoxidizing. During casting operations, air is injected through meltingburner 52 to maintain a positive oxidizing atmosphere in the smelter.

In order to melt the burden in the smelter, the following procedure isfollowed. As the rate of reduction decreases, the batch temperaturerises indicating the reduction process is nearing completion. Fuel gasinjection through melting burner 52 is increased to create additionalheat with an attendant increase in the melting rate. Oxygen, air, and/orfuel are injected through tuyeres 54, porous plugs, or other ceramicelements, to combine with the excess carbon both carried in the pelletsand added separately to the burden, and thus the batch is quicklymelted. Oxygen, fuel, and air are delivered to the interior of therotating vessel through rotary coupling 56.

When melting is completed, the movable fume hood 42 is retracted to theopen position. As soon as the fume hood clears the mouth of the smelter,the casting sequence begins. The slowly rotating smelter is tilted tothe casting position shown in FIG. 8, and the liquid contents are slowlypoured into a tundish where a partial slag/iron separation is made.Liquid iron and the remaining slag are diverted either into a pigmachine or into an iron shot-making vessel. Alternatively, the moltenmaterial is simply dumped into a prepared sand mold pit for cooling. Thecasting pit is hooded to divert casting dust and fumes into anafterburner bypass duct leading to the process dust collector 58.

Upon completion of the casting sequence, the inclined rotary reductionsmelter 38 is returned to charging position and the batch chargingprocess is repeated.

Processing time and temperature ranges for each operating sequence areapproximately as follows:

The smelter is in the casting position for about 5 to 10 minutes atnormal operating temperature.

The charging position requires about 5 minutes.

The smelting vessel is in the preheating position about the same lengthof time as the charging position, as preheating can occur duringcharging.

The smelting vessel is in the operating position

about 5 to 10 minutes for the batch temperature to reach 600° C. whilethe atmospheric temperature within the vessel increases to 900° C.;

about 10 to 15 minutes while the batch temperature rises to 900° C.while the atmospheric temperature within the vessel increases to 1100°C.;

about 10 to 15 minutes for the batch to reach 1050° C. to 1150° C. atwhich temperature the batch is maintained until reduction is complete.

Depending on the pellet analysis of the particular batch, the time toprocess each batch will vary between 35 and 55 minutes to completereduction.

To continue the process to melt the batch or burden, the atmospherictemperature within the vessel is increased to the range of 1500° C. to1650° C. by utilizing an oxy-fuel melting burner 52 and optionalinjection of oxygen and/or fuel under the burden within the smelter.Melting is completed in 15 to 20 minutes depending on the composition ofthe pellet batch.

Thus, it is seen that one entire sequence of batch operation requiresbetween 35 and 55 minutes to complete reduction of the pellet batch,plus an additional 15 to 20 minutes to complete the melting procedure,if melting should be the operating mode.

It is well known in the art that it is difficult to maintain a positiveatmospheric seal with rotating furnaces to prevent gas leakage from suchvessels. In any case where heavy metal fumes may be expelled from theprocess, it is vital that gas leakage to the surrounding work area beprevented. Semi-positive sealing means as provided by metal-to-metalslip-seals, or labyrinth-type seals are expensive to construct andmaintain. This invention utilizes an air gap between the lip of theinclined rotary reduction smelter 38 and the movable fume hood 42 toprovide limited air intake, and thus provide a positive gas seal. Theair intake gap is approximately 3 mm to 6 mm of space between therotatable vessel 38 and the stationary fume hood 42 in which a negativepressure is maintained. Pressure kicks and pressure pulses are avoidedby predrying the green pellets to prevent the sudden release of steamwithin the smelter furnace. The movable fume hood 42 is provided withboth temperature and oxygen sensing instruments which provides processcontrol data. Both the retractable burner 46 and the melting burner 52are controlled by output signals from fume hood sensors according to themode of operation.

When the movable fume hood 42 is returned to the operating positionillustrated in FIG. 5, a positive face-to-face seal is provided betweenthe top of the fume hood and the uptake duct 60 leading to theafterburner 30.

Hot process gases are sensed for free oxygen content while passingthrough the movable fume hood 42 and the afterburner combustion airblower 64 is activated to provide the afterburner air requirements tocomplete combustion of combustable gases as well as to reoxidizevolatile metals. A pilot burner 66 is provided in the afterburner toinsure ignition should the refractory temperature be below the necessaryignition temperature as it is in the case of starting up the system. Anexcess air damper 68 is employed to provide excess combustion air pluspartial gas cooling. The afterburner 30 contains a gas-to-air heatexchanger 28, from which drying air at 300° C. is generated for use inthe pellet dryer 26. However, depending on the local cost for naturalgas, the option to use natural gas for pellet drying may be choseninstead of using a heat exchanger in the afterburner.

Gas cooler 70 follows the afterburner 30 to accomplish gas cooling byconventional means and equipment to reduce the gas temperaturesufficiently to allow the process dust collector 72 to be ofconventional fabric bag-type.

The process of this invention includes the mechanical diversion ofcooled waste gases from the gas cooler 70 to one of three separatecompartments of the baghouse 72. The process dust collector header 74diverts the gas stream to one of the three baghouse compartments asrequired by the mode of smelter operation. During preheating anddevolatilization of the admixed carbon in the pellets, dust and gasesfrom the smelter are burned, cooled, and diverted to compartment A inthe baghouse 72. When reduction begins and zinc, lead, cadmium, andalkali fumes begin evolving, the waste gas streams are diverted tocompartment B and then to compartment C to take advantage of thevariable distillation effect of the batching process and to enhance theenrichment of the recollected zinc oxide and lead oxide dust.Recollected baghouse dust is removed from each baghouse compartmentindependently, and dust collected in compartment A is recycled back tomixer 22, where it is reblended with other feed materials for anotherpass through the smelting process. Dusts collected in compartments B andC may or may not be recycled through the smelter a second time dependingon the dust composition and the market for such materials.

ALTERNATIVE EMBODIMENTS

FIG. 10 shows an alternative embodiment in which the pellets are passedthrough a rotary hearth direct reduction furnace 80 prior to being fedto the inclined rotary reduction smelter 38. Means are provided forbypassing the rotary hearth furnace if the pellets need not bepre-reduced.

In some instances of any embodiment described, the pellet dryer may notbe required in the operation of the process.

SUMMARY OF THE ACHIEVEMENTS OF THE OBJECTS OF THE INVENTION

From the foregoing, it is readily apparent that we have provided asmall-scale flue dust recovery plant that can be operated on adiscontinuous basis, without costly degradation to the systemsrefractories. Our invention provides economic application "on-site" atmini-steelmills for the purpose of recycling hazardous EAF flue dustinto recyclable, and/or non-hazardous materials, including directlyreduced iron pellets, or pig iron and slag, and a highly reconcentratedzinc and lead oxide dust/ore. In addition, the invention provides abatch process which selectively distills and recovers reconcentratedzinc oxide dust, lead oxide dust, and process gangue dust separately.The apparatus has sufficient operating flexibility to allow feedmaterial to be converted to solid-state directly reduced iron, or to bemelted to form liquid iron and slag, while distilling volatile heavymetals for subsequent recovery in an off-gas cooling and dust collectionsystem. It also provides means to convert hazardous waste flue dustwhich has no direct commercial value into marketable products andby-products, and non-hazardous waste materials that can be safelydisposed of according to the EPA EP Toxicity Regulations.

Lastly, our invention provides means to terminate the existence ofcertain flue dust and sludge materials as hazardous waste.

What is claimed is:
 1. Apparatus for treating dust and sludgecontaminated with heavy metals and heavy metal oxides, comprising;wastematerial storage means; a mixer; means communicating with said wastematerial storage means and said mixer for introducing said wastematerial, solid carbonaceous material, and an organic binder to saidmixer; a pelletizing device; means for introducing material from saidmixer into said pelletizing device; pelletizer discharge means; aninclined rotary reduction smelter vessel having a charging and pouringopening in one end thereof; means for introducing pellets from saidpelletizer discharge means to said rotary reduction smelter vessel;retractable burner means for heating the interior of said smeltervessel; means for rotating said smelter vessel about its inclined axis;and means for tilting said smelter vessel about a horizontal axis. 2.Apparatus according to claim 1 further comprising a fume hood adaptedfor sealing engagement with the charging opening of said smelter vessel.3. Apparatus according to claim 1 wherein said means for introducingpellets to said smelter vessel is mounted for reciprocable movement intoand out of the charging opening in said vessel.
 4. Apparatus accordingto claim 2, further comprising an afterburner mounted within a gas pipeat the exit of said fume hood.
 5. Apparatus according to claim 1,further comprising a pellet dryer communicating with said pelletizerdischarge means, said dryer having means therein for forcing heated airdownwardly and upwardly through pellets on a conveyor means passingtherethrough.
 6. Apparatus according to claim 1, further comprising ameans for collecting off-gases from said smelter vessel, gas cleaningapparatus engageable with said off-gas collecting means and means forpassing said off-gases through said gas cleaning apparatus.
 7. Apparatusaccording to claim 6, further comprising means for selectively returningmaterials removed from said off-gases to said mixer.
 8. Apparatusaccording to claim 1, further comprising means for sizing pelletsexiting said pelletizing device.
 9. Apparatus according to claim 1,further comprising a direct reduction furnace, means for introducingsaid pellets to said direct reduction furnace, and means for introducingdirecting reduced pellets from said furnace to said smelting vessel. 10.An inclined rotary reduction smelting vessel comprising:a steel shell; arefractory lining within said steel shell; a charging and pouringopening at one end of the vessel, said vessel being mounted for rotarymotion about a central axis and for tilting motion about a horizontaltilting axis.
 11. Apparatus according to claim 10 wherein said tiltingaxis is external to said smelting vessel.
 12. Apparatus according toclaim 10, further comprising a hood removably mountable over thecharging and pouring opening of said smelting vessel.
 13. Apparatusaccording to claim 10, further comprising a retractable burner adaptedto be inserted into said smelting vessel through an orifice in saidhook.
 14. Apparatus according to claim 10, further comprising means forinjecting gases and/or fuels into said rotary smelting vessel. 15.Apparatus according to claim 14, wherein said means for injecting gasesis selected from the group comprising tuyeres, porous plugs, and ceramicelements.
 16. Apparatus according to claim 13 wherein said burner is awater cooled, retractable, oxy-fuel burner adapted to be progressivelypositioned at variously heating locations, and withdrawn into a movablefume hood during casting and charging operations.